1. Field of Invention
The present invention relates to a planarization method. More particularly, the present invention relates to a planarization method using anisotropic wet etching.
2. Description of Related Art
Dielectric material is used to isolate metal lines in the multilevel interconnects process to prevent shorts from occurring between two metal lines. Dielectric material used between two layers of metal lines is called intermetal dielectric (IMD). Because the surface of wafer is rugged after metal lines are formed thereon, the dielectric material subsequently deposited is also rugged. Therefore, the flatness of the IMD is the determining factor for the patterning of vias therein and metal wires thereon.
The planarization process is the key step for ensuring that high-density lithography can be performed, because light scattering problems can only be avoided in exposure steps when an IMD with sufficient flatness is provided. Therefore, when the IMD is sufficiently flat, a precise pattern-transferring step can be performed. Chemical mechanical polishing (CMP) is the technique that provides global planarization in current semiconductor processing. The CMP technique involves using a reagent to form a chemically altered layer on the non-planar surface of the material to be polished, followed by a mechanical removal of the chemically altered layer from the underlying bulk material.
The polishing slurry or the reagent used in a CMP process consists of a solvent and abrasive particles dispersed in the solvent. The solvent of the slurry chemically depletes, loosens, or modifies the composition of the material to be removed. The highly abrasive particles in the slurry, in combination with the rotating polishing pad, then physically remove the chemically modified unwanted material and polish the underlying surface. Since the abrasive particles in the polishing slurry are structurally very hard, scratches are easily induced on the surface of some materials during the CMP process. The problem of bridging is then likely to occur in the subsequent process, which affects the reliability of the device. A wet cleaning step is additionally needed to remove the abrasive particles used during the CMP.
The present invention provides a planarization method using anisotropic wet etching. According to one preferred embodiment of this invention, this method can be applied to planarize an insulating layer with an uneven surface on a substrate. H2SO4, H3PO4, HF and H2O are mixed to form an etching solution. The substrate is placed into the etching solution to make the etching solution pass the surface of the insulating layer at a flow rate to etch the insulating layer. After a period of etching time, the insulating layer with a more planar surface can be obtained.
The concentrations of the H2SO4, H3PO4, and HF are respectively about 98% wt., about 85% wt., and about 1% wt. The volume ratio of H2SO4 and H3PO4: HF is about 50-100:1, and the etching rate of the etching solution to an insulating layer with a planar surface is about 50-80 xc3x85/min.
The invention also provides a planarization method using anisotropic wet etching. A dummy pattern is formed on a region that has a lower pattern density to minimize the etching rate""s difference of regions having different pattern density.
According to another preferred embodiment of this invention, a first insulating layer is on a substrate, and the first insulating layer has large trenches and small trenches therein. A second insulating layer is conformably formed on the first insulating layer, and a thickness of the second insulating layer is about the same as a depth of the large and the small trenches. The second insulating layer is patterned to form protrusions in the large trenches, and a distance between the neighboring protrusions is about the same as the width of the small trenches. H2SO4, H3PO4, HF and H2O are mixed to form an etching solution. The substrate is then placed into the etching solution to make the etching solution pass the surface of the first and the second insulating layer at a flow rate to etch the first and the second insulating layers.
The concentrations of the H2SO4, H3PO4, and HF are respectively about 98% wt., about 85% wt., and about 1% wt. The volume ratio of H2SO4 and H3PO4:HF is about 50-100:1, and the etching rate of the etching solution to an insulating layer with a planar surface is about 50-80 xc3x85/min.
This invention utilizes an etching solution having different flow rate on a thin film having a rugged surface to etch the protrusions at a larger etching rate then the recess. Hence, the rugged surface of the thin film is planarized.
It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed.